The present invention relates to a bar code reader for scanning an area rather than a single line. In particular, the present invention relates to varying the rate at which the reader sweeps across the area depending on the sweep direction. This feature improves the reader's ability to read bar codes moving past the reader and permit increases in the speed at which the codes move past the reader.
Many applications such as industrial applications require bar code readers to search for bar codes passing the scanner with the bars and associate spaces of the bar codes having substantially vertical orientations (i.e. a picket fence orientation). To read these bar codes, the reader needs to scan an area instead of a single line. Thus, the reader must be capable of scanning in a raster pattern to cover the area. To provide the ability to scan a raster pattern as illustrated in FIG. 16, typical readers sweep a light beam along one axis (e.g. horizontal) while the raster angle of the beam is increased and decrease at a constant rate to produce a sweeping light beam which moves between the top and bottom of the raster pattern as illustrated in FIG. 16.
The problem with bar code readers which produce a raster pattern using the method discussed above is the limitation this method places on the speed at which bar codes can move past the reader. More specifically, referring to FIG. 16, if a bar code B is moving past the reader and enters the top-most portion of the raster pattern area when the light beam is at point P and sweeping downward toward the bottom of the area, the light beam must continue sweeping downward until reaching the bottom and then sweep upward toward the top of the area. (A similar situation occurs when a bar code is moving past the reader and enters the bottom-most portion of the raster pattern area.) Accordingly, to assure accurate reading of all codes which pass through the raster pattern area, the maximum speed of a conveyor moving an article having a bar code must be set so that if the bar code enters the top-most portion of the area when the light beam is beginning its downward sweep just below the bar code, the bar code does not exit the area before the light beam sweeps down to the bottom of the area and then back up through the top most part of the area.
The following example illustrates the limitations the method discussed above places on conveyor speeds. Assuming five (5) scans through a code are recommended for accurate reading, 0.3 degrees per scan cycle (i.e. light beam sweep from one side of the raster pattern area to the other side and back), and a thirty (30) degree raster angle, 200 scan cycles are required for the light beam to sweep from the top of the area to the bottom of the area and back. Using a typical bar code reader which can scan at about 150 scan cycles per second, 1.33 seconds are required for the reader to scan the area from top to bottom and back. In other words, a conveyor moving at 20 feet per minute will travel 5.3 inches during this time period. Thus, to read a bar code entering the top five (5) scan lines just after the light beam begins sweeping downward below the bar code at a conveyor speed of 20 feet per minute, the reader would have to scan an area about 5.3 inches wide.
In view of the problems discussed above, it would be desirable to provides for a scanner using a sweeping scheme which reduce the time required to scan a raster pattern area from the top to the bottom and back without impairing bar code reading accuracy.